This invention relates to surface-emitting heterojunction laser/LED structures.
The importance of vertical laser emission for optoelectronic integration was recognized by, for example, K. Iga et al, 9th IEEE International Semiconductor Conference, 1984, Rio de Janeiro, Brazil, paper D3, p 52. Vertical laser emission is particularly important in constructing large emitting areas which can be made to have narrow beam angles and high power outputs. Several types of surface-emitting laser are known. SpringThorpe et al, International Electron Devices Meeting, (1977) Washington, D.C., p 571 discloses a standard double heterostructure cavity transverse to current flow. The cavity is electrically pumped over most of its length and two additional mirrors are used to divert the laser beam towards the device surface. K. Iga et al, Electronics Letters, 19, #13 (1983) p 457 disclose a surface-emitting laser having a cavity perpendicular to the surface but pumped over a short length of the cavity by a pn junction co-planar with the surface. Ito et al, Electronics Letters 20 #14 (1984) p 577, elaborate on the Iga structure by elongating the cavity and introducing additional pumping along its length by a diffused homojunction.
In my co-pending patent application Ser. No. 701,839 filed Feb. 10, 1985 there is described an alternative surface emitting device. The device has a columnar active region of one direct bandgap semiconductor and a surrounding confining region of a higher bandgap semiconductor. Contacts are made to the active and confining regions and a window is formed in the device in vertical alignment with the active region to permit light emission from the device. The semiconductors are doped to establish a pn junction within a carrier diffusion length of the heterojunction between the active and confining region, the pn junction extending the length of the active region. In use, light is emitted along the axis of the columnar active region in response to current passing radially across the pn junction.
Any of these surface emitting lasers can have an active region epitaxially grown to provide layers of varying composition and with layer thicknesses such that in operation distributed feedback is provided. If the feedback occurs within the active region, these devices are termed distributed feedback lasers (DFB), whereas outside the active region they are generally referred to as distributed Bragg reflectors (DBR), see for example, IEEE Spectrum December, 1983, page 43. Bragg distributed reflectors for surface emitting lasers are described by Ogura et al, "GaAs/Al.sub.x Ga.sub.1-x As Multilayer Reflector for Surface Emitting Laser Diode", Japanese Journal of Applied Physics, volume 22, No. 2, February 1983, pp. L112-L114, while by Ogura et al, "Distributed Feed Back Surface Emitting Laser Diode with Multilayered Heterostructure", Japanese Journal of Applied Physics, Volume 23, No. 7, July 1984, pp L512-L514 demonstrates the actual application to a laser device.
The Ogura et al laser device has multilayer semiconductors formed within the active region and at opposed ends of the active region. The Ogura et al device is constructed by fabricating a multilayer of GaAs and Ga.sub.0.7 Al.sub.0.3 As and then diffusing zinc laterally to establish a pn homojunction. The zinc diffusion acts to mix the GaAs and Ga.sub.0.7 Al.sub.0.3 As on one side of the homojunction. For layers of equal thickness, the composition obtained on mixing is Ga.sub.0.85 As.sub.0.15 As. This alloy has a bandgap which is lower than GaAs and higher than Ga.sub.0.7 Al.sub.0.3 As. However the pumping or injection efficiency of a laser is exponentially related to the bandgap difference at a heterojunction, current carriers being directed from the material of larger bandgap to the material of smaller bandgap. Consequently in Ogura et al only the alternate layers can be pumped and the maximum bandgap difference is limited by the high bandgap material on one side of the homojunction being mixed with a low bandgap material.